Nonwoven cable wraps consist, as a rule, of a support layer of strong, thin nonwoven fabric coated on one side with a swelling medium, usually a powder, which upon contact with water swells abruptly to many times its initial bulk and consists, for example, of the cross-linked sodium salt of a polyacrylic acid. Such a swelling medium is usually referred to as a "superabsorber."
Water in telephone cables can lead to the failure of entire distribution networks; in the case of fiber cables it can lead to severe attenuation of the transmission output.
In plastic-insulated medium and high-voltage cables, moisture, especially in combination with contamination or air bubbles in the insulating structure and sufficiently great field strength, can cause so-called "water treeing" leading to the destruction of the insulation layer.
If the outer jacket of cables is damaged, for example when they are laid or during later road work, water can penetrate into the cable structure and propagate over long distances along the axis, especially in the shielding in power cables.
To prevent this, that is, to limit the local damage, is the purpose of a swellable cable wrap. In the case of damage, upon the entry of water, a blocking effect is produced by the rapid swelling of the wrap by the swelling medium and the resultant swelling pressure closing up voids and preventing the water from spreading out along the axis. The damage remains limited to a short distance of 1 to 2 m. For this purpose it is necessary for the reaction time of the swelling wrap to be as short as possible.
In power cables in the medium and high voltage range it is also necessary, depending on the principle of construction, that corresponding cable wraps which are disposed between the outer conductive layer and the shielding, be treated to make them electrically conductive. A permanent electrical contact between the shielding and the outer conductive layer must be assured. Such contact can be assured by the incorporation of conductive carbon black when making the wrap.
The wraps described can be either applied continuously lengthwise or wound with an overlap. Depending on the cable diameter and construction, the width of the band can be between 10 and 300 mm. Special wraps outside of this range are also possible. The thickness of the wraps can vary between 0.1 and 1 mm.
In the manufacture and handling of the wrap, to prevent the superabsorber from coming loose due to bending and creeping or to mechanical influences, coverings are used on the superabsorber, which are made of gossamer-thin nonwoven material. Also in the interest of minimizing health hazards in the workplace the superabsorber is covered with a thin nonwoven during the manufacture of such cables. This is described in the German periodical, "Drahtwelt," 2, 1989, "Quellvliesstoffe fur langswasserdichte Kabelkonstruktionen" (swelling nonwovens for longitudinally water-tight cable designs) page 14, left and right columns.
Such covering, however, has the disadvantage that the free swelling of the superabsorber is considerably hampered, since the free space available for the swelling is reduced by the covering.
In special cable designs--for example in so-called "chamber cables," in the fiber-optic cable field, or also in the case of great diameters of the shielding wires in power cables, free swelling is essential for the purpose of reliably closing up large voids. If this is not assured, the propagation of moisture lengthwise of the cable cannot be prevented with sufficient reliability.
Under the designation 3 E 111, manufactured by LANTOR BV in the Netherlands, a cable wrap is commercially available in which 15 to 40 g/m.sup.2 of superabsorber is fixed by means of a binding agent on a thin, high strength nonwoven support. The swelling agent is prevented from dropping off by applying large amounts in proportion to the amount of superabsorber to the nonwoven support. The superabsorber is pressed into the cable wrap by calander rolls to bond it more tightly.
This embodiment, which does not require any separate covering of the superabsorber and in which substantial amounts of the absorber are exposed, also has the disadvantage that the swelling is appreciably delayed upon entry of water; within 1 minute, far less than 90% of the final swelling thickness is reached since the swelling agent must first free itself from its embedded state in the binding agent by its own swelling pressure.
There are limits to reducing the amount of binding agent, because if too little binding agent is applied the superabsorber would powder off appreciably as soon as the product is transported or handled.